1. Field of the Invention
The present invention generally relates to an apparatus and methods for drilling, completion and rework of wells. More particularly, the invention relates to an apparatus and method for activating and releasing downhole tools. More particularly still, the invention provides an internal pressure indicator and locking mechanism for the downhole tool.
2. Description of the Related Art
In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a tubular string. After drilling to a predetermined depth, the tubular string and bit are removed, and the wellbore is lined with a string of steel pipe called casing. The casing provides support to the wellbore and facilitates the isolation of certain areas of the wellbore adjacent to hydrocarbon bearing formations. The casing typically extends down the wellbore from the surface of the well to a designated depth. An annular area is thus defined between the outside of the casing and the earth formation. During the completion process, this annular area is filled with cement to permanently set the casing in the wellbore and to facilitate the isolation of production zones and fluids at different depths within the wellbore.
Various downhole tools are used throughout the well completion process. One such downhole tool is a conventional under-reamer. Generally, the conventional under-reamer is used to enlarge the diameter of wellbore by cutting away a portion of the inner diameter of the existing wellbore. A conventional under-reamer is typically run downhole on a tubing string to a predetermined location with the under-reamer blades in a closed position. Subsequently, fluid is pumped into the conventional under-reamer and the blades extend outward to contact the surrounding wellbore. Thereafter, the blades are rotated through hydraulic means and the front blades enlarge the diameter of the existing wellbore as the conventional under-reamer is urged further into the wellbore.
The conventional under-reamer may also be used in a back-reaming operation. In the same manner as the under-reaming operation, fluid is pumped into the under-reamer and the blades are extended outward into contact with the surrounding wellbore. Thereafter, the blades are rotated through hydraulic means and the back blades enlarge the diameter of the existing wellbore as the under-reamer is pulled toward the surface of the wellbore. However, if the blades are not securely locked in place, the upward pulling of the under-reamer causes the blades to fluctuate between an inward and outward position, thereby creating an uneven hole.
A blade locking mechanism on a conventional under-reamer includes a mandrel with a taper. The mandrel is moved between a first and a second position by a spring. Typically, the mandrel uses the mechanical advantage of the taper to apply a force on a piston to keep the blades in the fully open position. The amount of taper on the mandrel is critical to reduce the coefficient of friction at the mandrel and blade interface. For example, if the taper on the mandrel is too small, the spring will be unable to pull the mandrel from the second position to the first position, thereby causing the conventional under-reamer to become immobilized downhole. On the other hand, if the taper is too large, the mechanical advantage of the mandrel is diminished, thereby reducing the force on the piston. In either case, due to downhole conditions, the coefficient of friction on moving parts can vary greatly, making this method of locking the blades open very unpredictable.
Typically, fluid pumped through the conventional under-reamer is used to move the mandrel from the first position to the second position. In the second position, the mandrel acts against the cam mechanism to open the blades. As the mandrel slides on a body of the conventional under-reamer toward the second position, a plurality of bypass holes are exposed in the body allowing some fluid to flow out of the conventional under-reamer resulting in a lower pressure in the conventional under-reamer. This lower pressure is used as an indicator to the operator that the blades are open because the mandrel is in the second position. There are several problems associated with the use of bypass holes as an indicator. One problem relates to the less positive indication. In this method, the bypass holes are exposed as the mandrel travels on the body, which may cause time flutter and throttling at low flow rates. Another problem is that this method permits a less accurate indication of the exact position of the blades during actuation of the conventional under-reamer.
There is a need therefore, for an under-reamer that includes a positive lock mechanism to ensure the blades remain open during a back reaming operation. There is a further need therefore, for an under-reamer that includes a locking mechanism that is predictable. There is a further need for an under-reamer that includes an indicator that permits an accurate indication of the exact position of the blades during actuation of the under-reamer.